Another term we have thrown around a lot that bears some explanation is Hotchkiss drive. This is a suspension layout very common on front-engine/rear-drive cars and trucks from the 1920s until the late 1970s and still used on many pick-up trucks and SUVs. Click here to read more about it.
Many rear-drive cars and trucks use a live axle at the rear — that is, the rear axle incorporates the differential and halfshafts into a single rigid unit that moves up and down with the rear wheels. Live axles are cheap and rugged and ensure that the camber of both rear wheels remains constant as the wheels move through their suspension travel. The main drawback of live axles is that they are heavy. Since their that mass is part of the vehicle’s unsprung weight, that’s detrimental to ride and handling.
A live axle must be located — that is, its movements in each plane must be limited so that the rear wheels remain firmly on the ground as much as possible — and it requires some means of transmitting any torque applied to the axle (whether by the engine or the brakes) to the body/frame.
There are two basic methods for transmitting axle torque:
- Closed driveshaft (torque tube): The first option is to enclose the driveshaft in a torque tube, connected rigidly to the axle housing and linked to the transmission via a single universal joint that allows the axle to move relative to the transmission without affecting the shaft’s rotation. Drive forces from the wheels are transmitted through the rigid torque tube to the transmission mount. Torque tubes were fairly common until the early 1960s (they were used by, among others, American Motors, Buick, and Chevrolet), but they fell out of favor because the torque tube adds significantly to unsprung weight.
- Open driveshaft: The second option is to leave the driveshaft open and use a universal joint on each end, allowing the axle to move relative to the driveshaft. Since the linkage between the axle and the driveshaft is now flexible, the driveshaft itself cannot transmit acceleration or braking torque. Instead, that chore must be performed by trailing arms (also known as radius rods) that connect the axle to the body/frame.
When the engine applies torque to the axle, that force tends to cause windup — that is, causing the axle assembly to twist along with the drive wheels. Severe axle windup can cause the entire axle to hop up and down on the rear springs, an effect known as axle tramp. Once the vehicle is actually accelerating, front-to-rear weight transfer also tends to cause the axle to squat. Similarly, when the brakes are applied, the rear axle tries to rise as weight shifts forward onto the front springs (an effect called brake dive), sometimes causing the rear wheels to break traction with the ground and hop. Meanwhile, in turns or on uneven pavement, lateral forces attempt to displace the axle assembly to one side. For the axle to do its job properly, it must be connected to the body with locating members — control arms or links — whose leverage resists all these forces.
Automakers have developed a variety of methods for locating a live axle. One approach, popular at GM for many years, uses four trailing arms, two above the axle and two below it, with the upper arms angled inward so that they resist lateral displacement of the axle. A three-link variation of that approach, used by Alfa Romeo for many years, uses a single triangular or T-shaped upper arm attached to the differential housing to perform the same function. Another common three-link layout, used by Buick in the 1960s, also uses two lower control arms and a single upper arm mounted next to the differential, but adds a lateral track bar (a Panhard rod) or parallelogram linkage (Watt’s linkage) to limit lateral motion.
Both the three-link and four-link layouts are reasonably effective, but the control arms and track bars make the rear suspension more complex and thus more expensive to build. A simpler alternative is Hotchkiss drive. In a Hotchkiss layout, the axle is suspended by a pair of longitudinally mounted semi-elliptical leaf springs that serve to locate the axle as well as support the weight of the body. Wheel forces are transmitted to the frame and/or body through the leading (front) portion of each spring, which also serves to resist squat and axle tramp. The trailing (rear) portion of each spring acts as a leading arm, resisting wheel hop under braking. The springs also resist any lateral axle motions. By making the springs perform multiple duties in this way, Hotchkiss drive is very simple and thus very cheap. Since it has few parts, it’s also very sturdy, which is useful for heavy-duty vehicles like trucks.
The drawback of Hotchkiss drive is that while the springs can perform all these various functions, they don’t necessarily do them well. The flexibility of a leaf spring limits its usefulness as a control link; if the spring isn’t very rigid, it will move or deform in response to the various forces on the axle rather than resisting them. Making the springs stiffer makes them more effective in controlling axle movement, but it also makes the ride firmer, sometimes uncomfortably so. The more powerful the engine, the greater the problem. Torquey engines like the big-block V8s of the muscle car era can exert so much force on the axle that the only ways to adequately limit axle movement are to (a) make the springs brutally stiff or (b) add auxiliary control arms (popularly known as “traction bars”) — and/or a Panhard rod and/or Watt’s linkage — to help control the axle, which is anathema to the whole rationale for using Hotchkiss drive in the first place.
One stopgap method, which Chrysler and some Studebakers used for many years, is to change the position of the axle on the springs. The spring rate of a leaf spring is proportional to its length. If you move the axle forward toward the leading ends of the springs, the front section of the spring will be shorter and thus stiffer, allowing it to better control axle tramp without making the ride harsher. The drawback is doing so effectively softens the rear portions of the springs (the sections aft of the axle assembly), making them less able to resist brake dive or wheel hop on deceleration. Powerful Chrysler cars of the sixties had good axle control on acceleration, but were prone to violent wheel hop on hard braking, occasionally with harrowing results.
Another stopgap, employed at various points by Chevrolet and Ford, among others, is to “stagger” the rear shock absorbers, mounting one ahead of the axle, the other behind it. In this way, the shock absorbers are made to perform double duty, resisting axle tramp. Staggered shocks can be reasonably effective for street cars, but may not be adequate for really high-powered applications like drag racing.
Whence the name “Hotchkiss drive?” The layout was first adopted in 1905 by the French firm of Hotchkiss et Cie. Few things in the automotive world are ever really new, however, and similar layouts had previously been used by other automakers, including Cleveland’s Peerless Motor Company. Nevertheless, the name has stuck, even if it’s not entirely accurate.